This application relates to three other patent applications directed to diffusion bonding processes for the preparation of composite high performance rotary anodes for X-ray tubes. These applications, which are incorporated by reference, are U.S. patent application Ser. Nos. 702,164 --Devine, Jr., filed Feb. 15, 1985, Ser. No. 702,160 --Devine, Jr., filed Feb. 15, 1985 and Ser. No. 702,161 --Devine Jr., filed Feb. 15, 1985.
Workers in the field of designing rotary anodes for conventional X-ray imaging systems have long recognized the advantages of utilizing graphite in such construction. It soon became evident that in using graphite there also exists the danger that when a metallic surface of tungsten, tungsten alloys, molybdenum or molybdenum alloys is in direct contact with graphite, reactions between the metallic surface and the graphite (during manufacture of the rotary target and/or during use thereof to generate the X-ray beam) lead to the formation of a brittle intermediate carbide layer. The patent literature proposes various anode constructions as solutions to this problem, for example, U.S. Pat. Nos. 3,660,053; 3,719,854 and British Pat. Nos. 1,173,859; 1,207,648 and 1,247,244.
Another patent, U.S. Pat. No. 3,890,521 (U.S. '521), expresses concern with the formation of tungsten carbide by reaction between a graphite disc, or carrier, and the tungsten target layer while accepting the in situ formation of a carbide layer of tantalum (or presumably of hafnium, niobium or zirconium). Both this patent and U.S. Pat. No. 3,710,170 are concerned with thermal stresses introduced in the rotary anode structure because of the difference in thermal expansion coefficients between tantalum carbide (U.S. '521) and the adjoining structure and between graphite (U.S '170) and the adjoining structure. However, in the case of U.S. '170, as well as in U.S. '521, certain metal carbide content is deliberately employed as part of the solder material. For example, in U.S. '170 it is proposed that a molybdenum-molybdenum carbide eutectic be prepared by placing graphite in contact with molybdenum and heating to about 2200.degree. C.
Still another concern is evident in British Pat. No. 1,383,557 wherein a solder layer of zirconium and/or titanium is employed to join graphite to molybdenum, tantalum or an alloy formed between two or more of tungsten, molybdenum, tantalum and rhenium. A carbide layer is formed between the graphite support and the solder layer. Particular temperature control and initial foil thickness are employed to insure survival of the solder layer.
The great variance in thought in the preceding prior art as to how to best join graphite to refractory metals, particularly tungsten, tungsten alloys, molybdenum and molybdenum alloys, shows how complex this problem has remained in the design of rotary anodes for conventional X-ray apparatus.
These varied solutions, to the extent they may be viable in conventional X-ray imaging systems, face a much more severe test in connection with the use of graphite members in X-ray tubes used in medical computerized axial tomography (C.A.T.) scanners. In the formation of images, a medical C.A.T. scanner typically requires an X-ray beam of from 2 to 8 seconds in duration. Such exposure times are much longer than the fractions-of-a-second exposure times typical for conventional X-ray imaging systems. As a result of these increased exposure times, much larger quantities of heat (produced as a by-product of the process of X-ray generation in the target region) must be stored and eventually dissipated by the rotating anode.
Graphite, which provides a low mass, high heat storage volume, remains a prime candidate, of course, for inclusion in rotating anode structures for C.A.T. scanner X-ray tubes, particularly when the graphite member functions as a heat sink from which heat is dissipated as radiant energy as is disclosed in U.S. Pat. No. 3,710,170 and U.S. Pat. No. Re. 31,568, rather than as support for the target anode layer.
One important consideration in the manufacture of a composite anode disc embodying a graphite member is the method by which the graphite is bonded to an adjacent tungsten, tungsten alloy, molybdenum or molybdenum alloy metallic surface. In spite of the favorable view taken of the presence of carbides of tantalum, hafnium, niobium, zirconium and of the eutectic of molybdenum carbide and molybdenum in U.S. '170 and/or U.S. '521, workers in the art view with alarm the formation of any layer of tungsten carbide or molybdenum carbide between the graphite member and an adjacent tungsten, tungsten alloy, molybdenum or molybdenum alloy surface to which the graphite must remain bonded. Formation of such a carbide layer is of particular concern, because of the propensity thereof for cracking. Cracking results in a reduction in heat flow from the metal surface to the adjacent graphite member and frequently will compromise the structural integrity of the anode.
In X-ray tubes used in C.A.T. scanners, the bulk temperatures of such anodes reach temperatures of 1200.degree.-1300.degree. C. in operation. At such temperatures, tungsten, tungsten alloys, molybdenum or molybdenum alloys readily form the undesired metal carbide. Thus, it has been considered particularly important for such rotary anodes to devise a joining procedure and anode structure in which the metallic surface is not permitted to react with the graphite and, even more important, that provision is made in the composite anode structure to prevent reaction from occurring between the metallic surface and the graphite during operation of the C.A.T. scanner X-ray tube.
Three reissue patents (U.S. Pat. Nos. Re. 31,369; Re. 31,560 and Re. 31,568) issued to Thomas M. Devine, Jr., describe a brazing procedure in which a layer of platinum, palladium, rhodium, osmium, ruthenium or platinum-chromium alloy is interposed between the metallic surface and the graphite body to which it is to be joined. Although a brazed region develops above and below the interposed layer, this layer itself survives to function as a barrier to carbon diffusion during operation of the X-ray tube. The aforementioned braze materials are characterized by their ability to react with tungsten, tungsten alloys, molybdenum, molybdenum alloys and also with graphite Because the reaction of the interposed layer with graphite can only proceed at a temperatures in excess of the temperatures that are reached by the rotating anode in service, even at the maximum service temperature an intermediate platinum layer, for example, will act as a diffusion barrier for carbon to prevent the passage thereof through the platinum, where it would be able to form the brittle tungsten or molybdenum carbide.
The use of alloys of platinum as an intermediate layer to join graphite to tungsten or tungsten alloy is disclosed in Gebrauchmuster No. 7,112,589 and the use of alloys containing platinum as an intermediate layer to join graphite to tungsten or molybdenum is disclosed in U.S. Pat. No. 3,442,006 (U.S. '006). In both of these inventions the process for joining requires that the intermediate layer be melted. An intermediate layer of any of the alloys proposed in U.S. '006 would fail as a diffusion barrier to carbon at X-ray anode operating temperatures.
In the practice of the inventions in the Devine patents the formation of the objectionable carbide is avoided provided that the brazing is accomplished quickly. At the brazing temperatures employed, which render the intermediate layer (e.g., platinum) molten, the intermediate molten layer can become saturated with carbon. By way of example, liquid platinum can, over a period of time at a temperature just above the eutectic temperature, dissolve up to about 16 atomic percent carbon. When tungsten or molybdenum is in contact with such a high carbon content liquid, carbide will form at the interface. The amount of time available for the carbon to dissolve in the liquefied braze layer is, therefore, important and if the assembly being brazed remains at a high temperature for too long a period of time, a thick layer of carbide can form, which layer is in danger of becoming cracked during cooling or handling. In the case of the use of platinum as the braze layer to affix molybdenum to graphite, a temperature exposure of about 1800.degree. C. for as little as about 5 minutes will result in a layer of molybdenum carbide about 0.003 inch in thickness.
Therefore, in the practice of the process disclosed in the Devine reissue patents, if brazing capability is available at the manufacturing facility to provide fast ramping to brazing temperature, holding for a short time and then cooling to below 1400.degree. C. in a brief time frame, carbide formatio.n is avoided. However, such ideal heating arrangements, which are commercially available, may not be accessible and it may be necessary to use a larger furnace. A problem that will occur when a number of rotary anode discs (typically 4 or 5 inches in diameter) are processed simultaneously in a furnace of high thermal mass is that each such disc tends to stay hot for a relatively long period of time and thick, cracked layers of carbide can form. Consequently, as an alternative to the aforementioned brazing method, it would be desirable to have a joining technique and anode composition, which can tolerate having the anode discs spend a finite length of time (e.g., minutes) at the joining temperature (and thereby permit the use of furnaces of high thermal mass) and the rotary anodes produced from such composites will be able to render high quality performance in the rigorous environment of the C.A.T. scanner X-ray tube.